The present invention is directed to toners, developers, and imaging processes, including a process for forming two-color images, and more specifically, the present invention is directed to a process for obtaining two-color images which in embodiments comprises charging an imaging member; creating on the member a latent image comprising areas of high, medium, and low potentials; developing the low areas of potential with a positively charged colored developer composition referred to as discharge area development (DAD); subsequently, developing the high areas of potential with a negatively charged black developer composition referred to as charged area development (CAD), and transferring the developed images to a substrate, and optionally permanently affixing the image to the substrate. The positively charged color developer composition, which in embodiments of the present invention is comprised of a negatively charged magenta toner comprised, for example, of resin and charge additive, or a mixture of charge enhancing additives of, for example, distearyl dimethyl ammonium methyl sulfate (DDAMS) and cetylpyridinium chloride, and surface additives of silica, metal salts of a fatty acid, and an aluminum complex, such as BONTRON E-88.RTM., can be selected for the development of the low areas of potential; thereafter, the high areas of potential can be developed with a negatively charged black developer composition comprised of a positively charged black toner comprised, for example, of a resin, carbon black and a charge enhancing additive, transferring both the color and black developed images to a substrate, and permanently affixing the images to the substrate by, for example, heat or a combination of heat and pressure In one development process embodiment of the present invention, there can be selected the toners and developers as illustrated herein for the methods as illustrated in U.S. Pat. No. 4,078,929, the disclosure of which is totally incorporated herein by reference. In process embodiments of the present invention, the photoresponsive imaging member can be negatively charged, positively charged, or both, and the latent image formed on the surface may be comprised of either a positive or a negative potential, or both. In one embodiment of the present invention, the image comprises three distinct levels of potential, all being of the same polarity. The levels of potential should be well differentiated, such that they are separated by at least 100 volts, and preferably 200 volts, or more. For example, a latent image on an imaging member can comprise areas of potential at -800, -400, and -100 volts. In addition, the levels of potential may comprise ranges of potential. For example, a latent image may comprise a high level of potential ranging from about -500 to about -800 volts, an intermediate level of potential of about -400 volts, and a low level ranging from about -100 to about -300 volts. An image having levels of potential that range over a broad area may be created such that gray areas of one color are developed in the high range and gray areas of another color are developed in the low range with 100 volts of potential separating the high and low ranges and constituting the intermediate, undeveloped range. In this situation, from 0 to about 100 volts may separate the high level of potential from the intermediate level of potential, and from 0 to about 100 volts may separate the intermediate level of potential from the low level of potential. When a layered organic photoreceptor is employed, preferred potential ranges are from about -700 to about -850 volts for the high level of potential, from about -350 to about -500 volts for the intermediate level of potential, and from about -80 to about -180 volts for the low level of potential. These potentials can vary depending, for example, upon the type of imaging member selected, and the like.
One advantage associated with the toners and processes of the present invention is the ability to generate high quality two-color images in a single development pass, particularly as a result of the absence of interaction between the colored, excluding black, and the black developers in embodiments of the present invention Other advantages associated with the present invention include the provision of a triboelectrically stable positive charging colored developer comprised of a negative charging color toner, which enables the generation of high quality images subsequent to development, that is images with substantially no background deposits. Moreover, the toners and developers of the present invention in embodiments can be selected as extended color palettes, reference for example U.S. Pat. No. 5,370,962, the disclosure of which is totally incorporated herein by reference.
Processes for obtaining electrophotographic, including xerographic, and two-colored images are known. In U.S. Pat. No. 4,264,185, the disclosure of which is totally incorporated herein by reference, there is illustrated an apparatus for forming two color images by forming a bipolar electrostatic image of a two-color original document on a photoconductive drum. A first developing unit applies a toner of a first color and polarity to the drum, and a second developing unit applies a toner of a second color and polarity to the drum to form a two color electrostatic image which is transferred and fixed to a copy sheet. A bias voltage of the first polarity is applied to the second developing unit to repel the toner of the first color and prevent degradation of the first color toner image. A bias voltage of the second polarity is applied to the first developing unit to prevent contamination of the first color toner with the second color toner.
In U.S. Pat. No. 4,308,821, the disclosure of which is totally incorporated herein by reference, there is illustrated a method and apparatus for forming two-color images which employs two magnetic brushes.
Further, U.S. Pat. No. 4,378,415, the disclosure of which is totally incorporated herein by reference, illustrates a method of highlight color imaging, which comprises providing a layered organic photoreceptor having a red sensitive layer and a short wavelength sensitive layer, subjecting the imaging member to negative charges, followed by subjecting the imaging member to positive charges, imagewise exposing the member, and developing with a colored developer composition comprising positively charged toner components, negatively charged toner components and carrier particles. In U.S. Pat. No. 4,430,402, there is illustrated a two-component type dry developer for use in dichromatic electrophotography, which comprises two kinds of developers, each of which consists of a toner and a carrier. Dichromatic images are formed by developing both positively and negatively electrified electrostatic latent image successively with toners different in polarity and color from each other, wherein one carrier becomes positively charged by friction with either of the two toners while the other carrier becomes negatively charged by friction with either of the two toners. In addition, U.S. Pat. No. 4,640,883, the disclosure of which is totally incorporated herein by reference, illustrates a method of forming composite or dichromatic images, which comprises forming on an imaging member electrostatic latent images having at least three different potential levels, the first and second latent images being represented, respectively, by a first potential and a second potential relative to a common background potential. The first and second images are developed by a first magnetic brush using two kinds of toners, at least one of which is magnetic, and both of which are chargeable to polarities opposite to each other with application to a developing electrode of a bias voltage capable of depositing the magnetic toner on the background potential area to deposit selectively the two toners on the first and second latent images, and to deposit the magnetic toner on the background potential area, while collecting the deposited magnetic toner at least from the background potential area by second magnetic brush developing means.
The process of charging a photoresponsive imaging member to a single polarity and creating on it an image consisting of at least three different levels of potential of the same polarity is described in U.S. Pat. No. 4,078,929, the disclosure of which is totally incorporated herein by reference. This patent discloses a method of creating two colored images by creating on an imaging surface a charge pattern including an area of first charge as a background area, a second area of greater voltage than the first area, and a third area of lesser voltage than the first area with the second and third areas functioning as image areas. The charge pattern is developed in a first step with positively charged toner particles of a first color and, in a subsequent development step, developed with negatively charged toner particles of a second color. Alternatively, charge patterns may be developed with a dry developer containing toners of two different colors in a single development step. Also of interest with respect to the trilevel process for generating images is U.S. Pat. No. 4,686,163, the disclosure of which is totally incorporated herein by reference.
Developer compositions with charge enhancing additives, which impart a positive charge to the toner resin, are known. Thus, for example, there is described in U.S. Pat. No. 3,893,935 the use of quaternary ammonium salts as charge control agents for electrostatic toner compositions. Also of interest is U.S. Pat. No. 4,221,856, which discloses electrophotographic toners containing resin compatible quaternary ammonium compounds in which at least two R radicals are hydrocarbons having from 8 to about 22 carbon atoms, and each other R is a hydrogen or hydrocarbon radical with from 1 to about 8 carbon atoms, and A is an anion, for example sulfate, sulfonate, nitrate, borate, chlorate, and the halogens such as iodide, chloride and bromide. There are also described in U.S. Pat. No. 2,986,521 reversal developer compositions comprised of toner resin particles coated with finely divided colloidal silica. According to the disclosure of this patent, the development of electrostatic latent images on negatively charged surfaces is accomplished by applying a developer composition having a positively charged triboelectric relationship with respect to the colloidal silica.
Also, there are illustrated in U.S. Pat. No. 4,338,390, the disclosure of which is totally incorporated herein by reference, developer compositions containing as charge enhancing additives organic sulfate and sulfonates, which additives can impart a positive charge to the toner composition. Further, there is disclosed in U.S. Pat. No. 4,298,672, the disclosure of which is totally incorporated herein by reference, positively charged toner compositions with resin particles and pigment particles, and as charge enhancing additives alkyl pyridinium compounds. Additionally, other documents disclosing positively charged toner compositions with charge control additives include U.S. Pat. Nos. 3,944,493; 4,007,293; 4,079,014; 4,394,430, and 4,560,635, which illustrates a toner with a distearyl dimethyl ammonium methyl sulfate charge additive.
Illustrated in copending application U.S. Ser. No. 528,927 now U.S. Pat. No. 5,536,680, filed concurrently herewith, the disclosure of which is totally incorporated herein by reference, is an imaging process which comprises (1) charging an imaging member in an imaging apparatus; (2) creating on the member a latent image comprising areas of high, intermediate, and low potential; (3) developing the low areas of potential with a first developer comprising carrier and a first negatively charged toner comprised of resin, the cyan pigment Pigment Blue 15:3, Color Index number 74160:3, CAS Number 147-14-8, a mixture of charge enhancing additives, and surface additives; (4) developing the high areas of potential with a second developer comprising carrier and a second black toner comprised of resin, pigment, and a charge enhancing additive that enables a positively charged toner; (5) transferring the resulting developed image to a substrate; and (6) fixing the image thereto.